1. Field of the Invention
The present invention relates to the control of an inverter for driving an induction motor at variable speeds by varying the drive frequency of the motor, and relates particularly to an inverter control apparatus for air conditioning systems using a heat pump cycle to provide heating and cooling.
2. Description of the Prior Art
In general, compressors used in air conditioning systems for the home, small office, or small retail establishment are driven by induction motors. The slip frequency, i.e., the difference between the drive frequency and rotational frequency, of an induction motor tends to fluctuate under load, and thus the efficiency of the motor will vary.
To compensate for this variation while maintaining high motor efficiency, vector control methods, whereby the load is calculated from the motor current, have been proposed as a replacement for the conventional voltage-frequency (V/F) control methods for obtaining an operating voltage proportional to the operating command frequency.
An example of a high efficiency control method incorporating vector control technologies is shown in FIG. 1. This method is based on the "quieting technique for inverter scroll package air conditioners" described in "Reitoshi" (a Japanese journal on refrigeration), June 1992, pp. 50-55. The AC power supply 6 for the induction motor 1 which drives the compressor is first converted to a DC power voltage by the diode bridge 5 and a smoothing capacitor 4. The DC power voltage and frequency are then adjusted by the three-phase PWM inverter 2 to control and vary the speed of the induction motor 1.
It is often found difficult to install rotation detectors on air conditioning compressors. Therefore, in order to detect the motor speed, by calculation using an induction motor constant, and drive the motor with high efficiency, at least two current levels must be detected for the induction motor 1.
The excitation current and torque current calculator 101 detects motor currents Iu and Iv using current sensors 3u and 3v to obtain the excitation current I1d and torque current I1q, respectively. The detected torque current I1q is roughly proportional to the slip frequency fs, which can therefore be calculated using the torque current I1q and the motor constant. The command frequency can be adjusted to compensate for the slip frequency fs, and thereby maintain the motor speed at a controlled constant level.
Thus, the slip frequency is calculated by the slip frequency control means 7, and the motor speed is corrected so that the induction motor 1 rotates at a level equal to the rotational command frequency fref output by the heat pump cycle control means. Referring to FIG. 1, the adder 8 adds the rotational command frequency fref from the heat pump cycle control means and the value of a correction frequency to obtain the actual command frequency f1 for the three-phase PWM inverter 2.
Identifying the primary resistance, that is one motor constant, is accomplished by the primary resistance identifier 102 to obtain a more precise primary resistance value based on the difference between the excitation current command I1*d, obtained by the current minimizing control means 103, and the detected excitation current I1d. The actual primary voltage is then determined by the primary voltage compensator 104 based on the primary resistance obtained, the excitation current command I1*d, the torque current I1q, and the actual command frequency. The actual primary voltage is then supplied to the three-phase PWM inverter 2. The current supply is thus controlled, based on the rotational command frequency fref from the heat pump cycle control means, to provide the minimum current level required to keep the induction motor 1 operating at the command speed. Thus, by minimizing the current supply, efficiency of the motor improves approximately 10% at a 1.1 kgm motor torque. Furthermore, improvement has also been confirmed at other torque levels.
The method of the prior art described controls motor efficiency by obtaining the slip frequency from the torque current, an operation which requires a motor constant. Possible motor constants include the primary resistance, secondary resistance, and various inductances. These constants are known, however, to significantly fluctuate according to the temperature and other factors. This fluctuation results in a degradation of control performance such as an undesirable change in the rotational speed.
Furthermore, minimization of the motor current does not always yield a maximized motor efficiency. As shown in FIG. 24, when the voltage V1 applied to a motor increases, power consumption drops to the lowest level V1(Pmin) before the motor current drops to the lowest level V1(Imin).